Flywheel power supplies have emerged as an alternative to electrochemical batteries for storing energy, with many advantages including higher reliability, longer life, lower or no maintenance, higher power capability and environmental friendliness. Flywheel power supplies store energy in a rotating flywheel that is supported by a low friction bearing system inside a chamber. The chamber is usually evacuated to reduce losses from aerodynamic drag. A motor/generator accelerates the flywheel for storing energy, and decelerates the flywheel for retrieving energy. Power electronics maintain the flow of energy in and out of the system and can instantaneously prevent power interruptions, or alternatively can manage peak loads.
One way to support a flywheel for rotation at high speeds is with rolling element mechanical bearings such as ball bearings. The life of mechanical bearings is strongly influenced by the loads that these bearings must carry. To extend the life of flywheel systems using mechanical bearings, a magnetic bearing can effectively be used in combination with the mechanical bearings for the purpose of reducing the load on the mechanical bearings. In this arrangement, the flywheel typically rotates about a vertical axis and the mechanical bearings provide radial support while the magnetic bearing carries much of the flywheel""s weight axially. One such flywheel energy storage system is shown in FIG. 1. The flywheel system 30 includes a steel flywheel 31 that rotates inside an evacuated vessel 32. The interior of the vessel 32 is a chamber 41 maintained at a vacuum for reduction of aerodynamic drag. The flywheel 31, in this design, has an integrated motor/generator 33 that accelerates and decelerates the flywheel through cooperation with the outer diameter of the flywheel 31. The motor/generator 33 includes an outer laminated stator portion 34, motor/generator coils 42, stator flux return path 35 and a field coil 36 for operation. The flywheel 31 is supported for rotation on upper and lower rolling element mechanical bearings 37 and 39. These bearings 37, 39 are mounted in fixed upper and lower mounts 38 and 40. Load on the bearings 37, 39 is reduced through the use of an axial magnetic bearing 43, shown as an annular electromagnet with electromagnetic coil 44. The magnetic bearing 43 supports a majority of the weight of the flywheel 31 while allowing some desired amount of loading on the bearings 37, 39. The electromagnet is controlled either by using strain gauges, not shown, on the support structure that sense and control the bearing loading through use of a closed loop controller, or simply by use of a constant current power supply.
Unfortunately, this configuration of mechanical and magnetic bearing system is not optimal for allowing rotation to very high-speeds for efficiently storing large amounts of energy. There are several drawbacks. The rigid radial support provided by the mechanical bearings would cause the rigid body critical speeds to be encountered at a relatively high speed if the flywheel were operated to high speeds. Encountering these resonances at a high speed would impart severe loading on the bearings that would reduce their life and potentially be dangerous. Operation at high speeds, but below the rigid body critical speeds, causes high bearing loading from any flywheel imbalances because the flywheel is being forced to spin about a geometric center rather than the mass center.
Another major problem with operating a flywheel system to high speeds is that the loads on the bearings can significantly increase due to dimensional changes in the flywheel. The effect of high speed rotation is illustrated in FIG. 2. The dimensions of a flywheel 50 are shown at 52 for zero speed and at 51 for high speed rotation. The effect of the centrifugal stress is that the outer diameter expectedly grows by a radial increment 53 from the radial and hoop stresses. This growth does not change the bearing loading. The secondary result from this growth is that the flywheel shrinks by an axial increment 54 from Poisson ratio contraction. For axially thick flywheels, the shrinkage can be as much as 0.050 inches. Such a large length change will not only drastically load the mechanical bearings against each other but can also cause them to fail before achieving full speed. The flywheel can also expand and contract axially from temperature changes in the flywheel or surrounding structure. Heating from high power motor/generators is one potential cause for added bearing loads.
The increased bearing loading drastically affects the life of mechanical bearings. Bearing life is generally a cubic function of the load, so that a doubling of the load will decrease the life by a factor of eight. Further compounding the shortening of life from the increased axial loading is that for angular contact bearings, axial loading can be as much as 35 times more fatiguing to the bearing than an equivalent size radial load. This sensitivity varies based on the contact angle, number of balls, ball diameter and the axial thrust load applied.
Besides problems of axial bearing loading that occurs between the bearings during operation, use of a mechanical strain gauges to measure axial loading at a single bearing is not as sensitive as desirable for removing almost the entire bearing axial load, especially if the flywheel support structure is rigid. Likewise, applying a constant current to the coil cannot provide sufficiently accurate axial load removal for maximum reliable operating life.
The construction of flywheel systems that support a flywheel with mechanical bearings can also suffer significant damage from shipping and handling. The system shown in FIG. 1 has no power during shipping and hence the ball bearings must carry the full flywheel weight. The strain gage and load cell can become damaged and plastically deformed from impact loadings, especially if they were designed to be sensitive enough to maintain very low axial bearing loading. The bearings of this as well as other design systems could be easily damaged from impact loads such as simply setting the system down during transportation handling. The force generated from an impact can be several times the weight of the flywheel and can cause the balls to Brinnell indent the bearing races or cause the bearings to shift in position.
This invention provides a flywheel energy storage system that allows high-speed operation with use of mechanical rolling element bearings for flywheel support. The mechanical bearings provide radial support for a vertical axis flywheel but they allow it to be mechanically free or unrestrained in the axial direction. One or more magnetic bearings are used to carry the flywheel weight axially. The axial unconstraint by the mechanical bearings allows the flywheel to freely grow or shrink in axial length from Poisson Effect contraction that occurs when rotating to very high speeds and stress levels as well as from thermal expansions from motor/generator heating or other sources. Excessive axial loads applied from the bearings on each other are thereby prevented. The axial mechanical freedom also insures that the magnetic bearing carries all of the flywheel weight, thus dramatically extending the mechanical bearing lives. The life of rolling element bearings is generally a cubic function of the applied loads and axial loading on commonly used angular contact bearings is many times more fatiguing to the bearings than radially applied loads. Eliminating the axial loading from the flywheel greatly extends the bearing lives. Tandem multiple preloaded angular contact bearings can be used for the mechanical bearings. These bearing sets share the loads between several bearings, extending life, and are manufactured with the desirable minimum axial preload for longest term reliable operation. The axial preload is accurately built-in and does not change as the flywheel is rotated. Alternatively, the bearings can each be single bearing pairs that are preloaded using springs with a stiffness that is lower than the magnetic bearing. Therefore, the bearings maintain their near designed preload despite the axial position of the flywheel from the magnetic bearing support or changes in the flywheel dimensions.
The axial magnetic bearings can use permanent magnets, attached to the flywheel, that are arranged to be in vertical repulsion with stationary cooperating permanent magnets. This provides a completely passive axial magnetic bearing system. In another embodiment of the invention, the axial magnetic bearing uses an actively controlled electromagnetic coil. The coil is controlled using either flywheel axial acceleration or, more preferably, a position sensor. The coil can be used in a simple electromagnet or in a permanent magnet biased thrust actuator for higher lift force and/or lower power consumption. The use of an active magnetic bearing does not require magnets on the flywheel and has the potential for higher speed rotation. It also can be lower in cost, and not suffer from any demagnetization effects.
The high speed capability of flywheels in accordance with the invention is further facilitated by using radially compliant elements mechanically in series with the mechanical bearings. Providing for a lower radial stiffness allows the flywheel to traverse its rigid body resonance at a low speed. Above that speed, the flywheel spins about its mass center instead of the geometric center and radial loading on the bearing becomes much lower. The power loss from rotation can also be reduced. The flywheel can then smoothly and easily operate to higher speeds. Balance requirements for the flywheel can be significantly reduced, reducing costs and extending mechanical bearing life. In one embodiment, the radial support allows the flywheel to traverse its cylindrical rigid body critical resonance at a speed that is less than 25% of the normal fully charged operation speed.
The use of the radially compliant elements or springs with the mechanical bearings also has the effect of helping the flywheel to rotate stably while having axially free sliding connections to the bearings. Although not completely free, due to friction, the flywheel is essentially mechanically unrestrained by the mechanical bearings, so that it can move axially. Use of sliding joints where energy can be lost from frictional damping in the rotating object is well known in the field of high speed machinery to potentially cause problems with nonsynchronous rotordynamic whirl and is usually avoided. However, rotor whirl can be avoided if the foundation stiffness is made sufficiently low for the mass of the rotating object. In this case, the radially compliant elements or springs that allow the flywheel to spin above the rigid body resonance also help keep the system stable with the reduced stiffness.
In one aspect of the invention, the radially compliant elements are placed between the mechanical bearings and the flywheel such that they rotate with the flywheel. The result is that above the rigid body resonance, the radial springs simply deflect and the flywheel rotates about its mass center Because the springs rotate with the flywheel, the springs do not cycle with each revolution. The life of the springs in the flywheel systems are thus increased for longer-term reliable operation. The radially compliant element can be a radial spring such as a tolerance ring or alternatively a quill shaft.
In another embodiment, the radial springs do not rotate with the flywheel but have a fatigue life of greater than 5 billion cycles of radial deflection equal to the radial distance between the mass center and the geometric center of said flywheel. This provides for at least 1 year of continual rotation at 10,000 rpm. An even higher cycle life such as ten to twenty times higher is even more preferable to preferably last the life of the system. Besides reducing the rigid body resonances, the low radial stiffness can potentially allow the flywheel to spin smoothly through other vibration modes that may exist depending on the system construction.
The invention also makes the flywheel system significantly less prone to damage during shipping, handling and installation. The axially unrestrained condition of the flywheel in the mechanical bearings prevents the flywheel from axially impact loading the bearings with the weight of the flywheel when the system is set down. The radial compliant elements in series with the mechanical bearings prevent damage from radial impact loading. The result of the invention is a much more robust flywheel system employing mechanical bearings, a system that accounts for the axial flywheel dimension changes, a system that can rotate to higher speeds for storing more energy and a system that maximizes the life of the mechanical bearings by elimination of axial loading from the flywheel simultaneously with greatly reduced radial loading.
The invention provides for greatly increased mechanical bearing life of the flywheel system. To extend the bearing life even further, the flywheel can be designed to operate with a slower rotational speed, reducing the bearing fatigue cycles incurred. In one aspect of the invention, the flywheel is constructed primarily from steel instead of composite materials to provide a lower operating speed. The flywheel can also preferably be constructed with an increased diameter so its normal fill speed operation is at less than 25 krpm. Implementing a motor/generator that has an air core armature can also further reduce bearing loads. The air core armature provides high efficiency while reducing or eliminating non-circumferential force generation during operation. dr
The invention and its many attendant advantages will become more clear upon reading the following description of the preferred embodiments in conjunction with the following drawings, wherein:
FIG. 1 is a schematic elevation of a prior art flywheel energy storage system with a mechanical and magnetic bearing system;
FIG. 2 is a sectional elevation of a solid cylindrical flywheel illustrating the dimensional effects on the flywheel when rotated to high speed;
FIG. 3 is a schematic elevation of a flywheel energy storage system in accordance with the invention;
FIG. 4 is a schematic elevation of another configuration of flywheel energy storage system in accordance with the invention;
FIG. 5 is a bar chart comparing the mechanical bearing life versus flywheel diameter for use with the invention;
FIG. 6 is a schematic elevation of another flywheel energy storage system in accordance with the: invention;
FIG. 7 is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 8 is a schematic drawing of another flywheel energy storage system in accordance with the invention;
FIG. 9 is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 10 is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 11 is a schematic elevation of another flywheel energy storage system in accordance with the invention; and
FIG. 12 is a schematic elevation of another flywheel energy storage system in accordance with the invention.